In an electronic key system for a vehicle, activation means are installed in a door handle and a trunk lid of a vehicle. When an occupant handles (activates) these activation means, communication with an electronic key is started, an ID transmitted from the electronic key and an ID registered in a control unit are compared, and, when the comparison result showing the coincidence between the IDs is obtained, a door lock and the like are released.
In addition, an activation means is installed in an ignition knob of the vehicle; and, when the occupant handles the ignition knob after the occupant gets in a four-wheeled car, the communication with the electronic key is again performed, and the ID comparison for permitting the engine to be started is performed. When the comparison result showing the coincidence between the IDs is obtained, the engine is started.
In other words, in the electronic key system for a vehicle, the electronic key functions to release the door lock and permits the engine to be started.
When the above electronic key system for a vehicle is applied to a motorcycle, there is a possibility that the engine cannot be re-started if the driver allows the motorcycle to start moving subsequent to inadvertent loss of the electronic key following start-up of the engine, although there is no problem if the occupant becomes timely aware of the fact.
Further, in the system applied to a motorcycle, the main purpose is prevention of theft; and the security system is designed to be stopped after the authorized occupant completes the authentication and rides on the motorcycle. Accordingly, there is a possibility that, if the electronic key has been dropped while the vehicle is running, for example, this fact cannot be detected.
In the prior art, the transmission interval of the request signal transmitted from the control unit while the vehicle is running is constant, and the appropriate transmission timing cannot always be realized, depending on the traveling state. For example, if the electronic key is dropped while the vehicle is traveling fast, the warning is outputted after the vehicle has traveled a substantial distance. In this case, the occupant has to return a substantial distance back to the point where the electronic key has been dropped, and there is apprehension that the time and labor required to search for the key will be substantial.
If the request signal is transmitted at short intervals constantly regardless of the traveling state, such apprehension can be eliminated. However, the result is that the electronic key transmits the answer signal correspondingly frequently, the power consumption of the battery increases, and the necessity to change the battery in a short time arises.